In the process of manufacturing integrated circuits, after the individual devices such as the transistors have been fabricated on the silicon substrate, they must be connected together to perform the desired circuit functions. This connection process is called "metalization" and is performed using a number of different photolithographic and deposition techniques. As the technology has been developing, electro-chemical deposition or electroplating has become the preferred metalization process to deposit the bulk amount of conductive metal and metal alloys for interconnecting semiconductor devices.
Typically, the chemical bath used in the electroplating process is the most difficult parameter to control. In order to achieve successful deposition of metal into smaller and smaller, high aspect ratio features, such as semiconductor trenches or vias where the width is small compared to a large depth, different fluid additives must be added to the plating bath to enhance the electroplating filling capability. Each plating bath is used to process in excess of 1,500 wafers per batch of plating solution. The useful life of a batch of plating solution is determined by how quickly the additives are consumed during the plating process, how quickly the additives are destroyed by oxidation during processing, or how quickly detrimental by-products from the plating process accumulate. All these factors create unstable and changing formulations of the plating bath. At the end of the useful life of the batch of plating solution, it must be disposed of and a new batch of plating solution formulated.
In the past, the plating bath control method employed by the semiconductor industry was to monitor important components on an analysis bench outside the plating equipment. For example, for copper plating, the important components for an acid copper plating bath included analysis for copper ion, sulfuric acid, hydrochloric acid, other additives, and carbon by-product level. After analysis, the desired component concentration was maintained by adding a required amount of the individual components in a predetermined amount so as to maintain the bath in a predetermined, but relatively wide, range.
The analysis of the carbon by-products generally determined when the plating bath solution needed to be disposed of and a new batch formulated since there was no method of removing the carbon without also removing some of the other additives.
As the semiconductor moves into the use of materials which require greater use of plating processes and greater control over the chemicals being applied to achieve smaller and smaller semiconductor geometries, it has become more pressing that a solution be found to provide precise process control in semiconductor manufacturing.